1. Field of the Art
The present invention relates to a turbo molecular pump including a magnetic bearing system.
2. Prior Art
A turbo molecular pump including a magnetic bearing system is known. For example, a prior art turbo molecular pump having active magnetic bearings is shown in FIG. 4. As shown, the turbo molecular pump comprises an impeller 1, a stator 2 and a shaft 1a integrated with the impeller 1, and a motor 12 is provided on an intermediate portion of the shaft 1a. Active magnetic bearings 3, 4 are each provided as radial bearings, and active magnetic bearings 5, 6 are provided as axial bearings. Reference numerals 10, 11 in FIG. 4 denote emergency bearings, 7 and 8 radial displacement detectors and 9 an axial displacement detector.
While not illustrated definitely therein, 5-axis active magnetic bearings include four active radial magnetic bearings and one active axial magnetic bearing.
Further, a prior art turbo molecular pump having a single axis active magnetic bearing system is shown in FIG. 5. The magnetic bearing system of FIG. 5 includes a passive magnetic bearing 13 as a radial bearing, and the active magnetic bearings 5, 6 as axial bearings, and is further provided with the emergency bearings 10, 11 as in the case of FIG. 4.
Reference numeral 14 in FIG. 5 denotes a shaft end protruding into the high vacuum side, 15 an emergency bearing retainer and 16 a pump casing.
However, in case of 5-axis active magnetic bearings of FIG. 4, since the number of control axes is many, high manufacturing costs may result, and control itself is made complicated thus impairing the reliability thereof. Further, since the axial control bearings 5, 6 are positioned near a shaft end on the side opposite to the impeller 1 of the rotor, the axial magnetic bearings 5, 6 must be disassembled to remove the impeller 1 from a pump for maintenance or like operations.
On the other hand, in the case of single axis active magnetic bearing of FIG. 5, since the emergency bearing 10 is positioned in a high vacuum side, impurities such as dust or the like which are produced when the emergency bearing is actuated on the high vacuum side may flow back to a vacuum vessel to be evacuated. Further, as in the case of FIG. 4, the axial magnetic bearings 5, 6 must be disassembled upon removing the impeller 1 from the pump.